Activated sludge wastewater treatment process

In a process for treating BOD-containing wastewater by contacting a wastewater influent with activated sludge in a reaction zone to form a mixed liquor, separating a sludge from the mixed liquor and recycling the separated sludge to the reaction zone as activated sludge, the improvement comprising

• introducing the wastewater and recycled activated sludge into a first aeration zone and homogeneously mixing the wastewater and recycled activated sludge therein primarily by injecting an oxygen-containing gas under conditions which produce a complete mix reaction in said first aeration zone and sufficient oxygen is provided by said oxygen-containing gas to meet, but not substantially exceed, the biological oxygen demand of the resulting mixed liquor such that the overall dissolved oxygen content thereof is substantially 0 and does not exceed about 0.5 mg/l, said oxygen-containing gas being introduced into the lower portion of said first aeration zone by a a plurality of fine bubble diffusers located in the bottom of said first reaction zone and having a flexible member made from an expandable, elastomeric material and including perforations having openings for producing bubbles which have a diameter of about 1 to about 4 mm, when said oxygen-containing gas is introduced under pressure into said diffusers, and are substantially uniformly dispersed throughout said first aeration zone;
• transferring the mixed liquor from said first aeration zone to a subsequent aeration zone wherein the mixed liquor is contacted with an oxygen-containing gas under conditions which produce a complete mix reaction in said subsequent aeration zone and sufficient oxygen is provided to produce an overall dissolved oxygen content of at least 0.5 mg/l;
• transferring the oxidized mixed liquor from said subsequent aeration zone to a settling zone wherein a supernatant liquid is separated from settled sludge;
• and
• recycling at least a portion of the settled sludge to said first aeration zone as recycled activated sludge.